PhD projects: 2014 cohort

Morphing mechanisms based on nonlinear composite elements

Chrysoula AzaStudent: Chrysoula Aza
Supervisors: Alberto Pirrera, Mark Schenk, Paul Weaver and Lorenzo Masia (NTU, Singapore)

In the field of robotics, there is a growing interest in the development of wearable devices - exosuits or exoskeletons - that assist or enhance human mobility. These devices show promise for use in medicine, for rehabilitation purposes, or, in work environments, to increase dexterity and/or improve performance. Over the past few years, a variety of such devices has been designed, but the use of conventional mechatronics still limits their performance. Current actuation systems are indeed not able to offer sufficient agility, dexterity or versatility compared to biological muscles. An actuation system that can simultaneously provide force control authority and power efficiency, whilst being lightweight, would be needed and remains a key requirement for the design of functional exoskeletons.

Building upon previous research on Variable Stiffness Actuators, this project sets out to explore the idea of merging modern electromechanical systems and morphing composite structures. We hypothesise that morphing structures' variable geometry, nonlinear stiffness characteristics and low weight may, in fact, be exploited to advance wearable robotic devices.

The main objective is to design, manufacture and test composite compliant components and actuators for wearable devices, suits and exoskeletons, with a particular focus on prosthetics and rehabilitation robotics.

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Polychromatic composite structures

Diego Bracho GarciaStudent: Diego Bracho Garcia
Supervisors: Annela Seddon, Richard Trask (University of Bath), Ian Hamerton and Karl Lymer (dstl)

Photonic crystals (PCs) are periodic ordered microstructures that exhibit distinct structural colour arising from the interaction of light with the periodically micro-structured dielectric and metallo-dielectric materials. Diffraction effects caused by the spatially periodic variation of the refractive index and dielectric function of these materials creates a photonic bandgap (PBG) in the visible spectrum, inhibiting or forbidding the propagation of light at certain wavelengths; analogue to the electronic bandgap in conventional semiconductors.

Through rational design and smart tuning of the PC periodicity it is possible to tailor the colour exhibited by these materials. Several different techniques have been studied for fabricating these photonic bandgap materials, including colloidal crystal templating and self-assembly, holographic patterning, and micromechanical drilling of a dielectric slab. In particular, colloidal self-assembly is a process of particular interest due to the comparative low cost and mild processing conditions. It is based on the fact that colloidal particles in suspension will spontaneously assemble into ordered structures under the appropriate conditions.

The use of smart and stimuli responsive materials in photonic structures allows reversible tailoring of colour by external physical or chemical stimuli, such as temperature changes, solvent infiltration, application of an electromagnetic field, etc. These novel materials are promising systems for applications in optoelectronics and photonics, photovoltaics, optical sensing, and colour display devices.

The focus of this study is to design and assemble stimuli-responsive composite PCs structures by self-assembly of polymeric and ceramic colloidal suspensions, in order to study their optical response and photonic band-gap tuning. The ultimate goal for this project is to produce large-scale tuneable colour display devices for applications in sensors, adaptive camouflage and cloaking systems.

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Nonlinear lattice structures: a numerical and analytical study on their stability

Max DixonStudent: Max Dixon
Supervisors: Alberto Pirrera, Isaac Chenchiah and Paul Weaver

Nonlinear systems can feature large displacements and variable stiffness, and also some distinctive characteristics such as multiple stable and unstable equilibria. These properties can be exploited to create multifunctional structures, i.e. structures that resist load, while simultaneously fulfilling other unconventional purposes. For example, a structure that is able to deform considerably, transition between stable states with different speeds, or indeed self-actuate or self-assemble, shows functional similarities to a conventional mechanism.

Inspired by the virus Bacteriophage T4 a bistable cylindrical structure mimicking the contraction of its tail sheath was designed. The tail sheath is a cylinder comprised of a series of molecular chains arranged as helices around the main axis. The contraction is achieved by altering the pitch of the helices. This mechanism is used by the virus to puncture the outer membrane of the host bacteria and to inject its DNA. We replicated the sheath kinematics with a device comprised of composite helices, forming a cylindrical lattice. By exploiting the interaction/interplay between pre-stress, structural coupling and geometric arrangement multistability is obtained. Furthermore, the structure is highly tailorable, as its stiffness properties, dimensions, and stability characteristics can be chosen to meet a variety of requirements. This versatility, as well as the multi-functionality of this structure, is reflected in the number of potential applications that we envisage. A few examples include:

  • Nonlinear springs/structural elements for damping and shock absorbing.
  • Deployable structures for space applications.
  • Robotic artificial muscles and exoskeletons.
  • Variable section conduits/nozzles.
  • Capsules for fluid delivery.

The main research challenge associated with this project is to demonstrate the viability of the concept of multifunctionality through structural nonlinearities. This can be done, for instance, by establishing a route to application/exploitation of the cylindrical lattice. An additional research challenge is demonstrating nonlinear/adaptive behaviour in lattices with non-cylindrical architectures. For instance, can hyperbolic, parabolic or even flat lattices exhibit nonlinear kinematics?

The aim of this project is twofold. The first objective is of applied engineering in nature and it is to take the lattice structure from concept to application. To do so, one or two of the aforementioned potential applications are explored to put the nonlinear lattice into context. Then, the device's design space is explored to identify benefits and unique features brought by the lattice's nonlinear kinematics. The models should be extended to capture all of the relevant behaviours. The second objective is of a more fundamental, engineering science nature. Can we create nonlinear/adaptive/deployable lattices with alternative architectures?

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Solvent-free, liquid processable bismaleimide-triazine resins

Rob IredaleStudent: Rob Iredale
Supervisors: Ian Hamerton and Carwyn Ward

Cyanate esters (CEs) are a family of addition cured, high performance, thermosetting polymers, which occupy a niche between high glass transition temperature tetra-functional epoxy resins and bismaleimides (BMIs). Cured CEs offer a combination of favourable thermal and mechanical properties. They are combined with BMIs as dielectric polymers in the microelectronics industry, a business which is predicted to reach a value of some $94 billion by 2017. A particularly well-established material in this area is the family of bismaleimide-triazine (BT) resins which is made up of multicomponent blends produced by Mitsubishi Gas Chemical Co. comprising a BMI and a CE combined with high boiling organic solvents to facilitate processing. BT resins have found broad application in printed circuit boards. The low dielectric constant, low loss properties and microwave transparency also make BT resins of particular interest in the fabrication of radomes, where a material with high dielectric constant or dissipation factor would otherwise result in attenuation of signal along with the structure of the radome itself being heated through conversion of electromagnetic energy to thermal energy. With the rapid advancement of these technologies there is a great demand for new BT resins in this field.

The biggest issue with the widespread application of BT resin systems is the poor processability of the uncured materials. In order to make the materials processable a mixture of high boiling solvents is used including dimethylformamide and dimethyl acetamide, which are potentially harmful to end users and to the environment on disposal.

The overall aim of this research, therefore, is to produce and characterise liquid processable BT resins and incorporate them into glass fibre reinforced composites (thus reducing the reliance on environmentally harmful solvents) suitable for use in the microelectronics and radome industries.

To achieve this several objectives must be achieved:

  • To blend commercially available monomers to achieve suitable rheological properties to infiltrate glass fibre reinforcement.
  • To optimise the processing steps to achieve the best network development and key performance characteristics (glass transition temperature, dielectric properties, thermal stability, etc.).
  • To produce glass fibres that are tailored to the resin system employed to yield the best interphase region and consequently the best interfacial strength.
  • To produce appropriate GFRP samples that can be tested for relevant physical and mechanical properties to demonstrate fitness for purpose in the technological application areas.

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Effect of high velocity oblique impact on carbon/epoxy laminates

Ashwin KristnamaStudent: Ashwin Kristnama
Supervisors: Michael Wisnom, Stephen Hallett, David Nowell (University of Oxford) and Adam Bishop (Rolls-Royce)

Ingestion of small and hard particles at high speed causes foreign object damage in gas turbine engines. Damage on composite structures may result in premature crack initiation, leading to failure due to harsh operating environment. The effect of foreign objects impacting on composite structures represent a high level of concern as damage can take various forms such as matrix cracks, delamination, fibre/matrix debonding, pull-out and fibre fracture. With the increasing usage of composites in aero engines, understanding the effect of FOD on component strength and structural integrity has become a critical activity, which is currently accounted for by expensive test programmes.

This project aims at greater understanding of the damage process such that component design can be improved and testing can potentially be reduced. Experimental investigations into the damage occurring during high speed impact events by small hard bodies will be conducted. Similarities to lower speed impact events and damage from machined notches will also be investigated, and the effect of different notch formation techniques on residual strength and specimen life will be assessed. Extensive modelling techniques for notched damage in composites will be employed to compare residual strength, specimen life and damage development against experimental results.

While much work has been carried out on damage development process from machined notches, this project mainly looks into damage development from impacted notches and relates the macroscopic damage and microscopic failure with other notch formation techniques. Potential benefit includes predicting damage development and subsequent behaviour of composite aerofoils and structures under FOD.

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Ultrasonic ray tracing in composite specimens with arbitrary shapes

Callum LanherneStudent: Callum Lanherne
Supervisors: Paul Wilcox and Fabrizio Scarpa

Non-destructive testing is an important step in the manufacturing process for any critical components, be it in aerospace, automotive or any number of industries. Ultrasound has been used extensively as a method for investigating the interior of structures for defects. In order to image such components the interior must be sampled, and the arrival times to each point calculated. In isotropic materials, such as aluminium, this is simple; the group velocity does not vary with direction, and so the path can be simply calculated, even in complex shapes such as curves. However when anisotropies are introduced, such as those in composite materials, this becomes less trivial. If a component has a curve, the fibres may follow that curve, so the local coordinate system is no longer translationally invariant. This leads to curved ray paths, and so a straight line calculation is no longer applicable.

Ultrasonic waves obey Fermat's principle - iterating over all possible paths, the path of least time will be the path that the ultrasound follows. Dijkstra's algorithm for shortest network paths then presents itself as a potential solution - by creating a network approximation, it should be possible to find the shortest path based on an anisotropic velocity in an arbitrary shape. This work investigates the validity of Dijkstra's algorithm for this purpose. The aim is to fully characterise Dijkstra's algorithm with respect to network distribution type, nodal density/separation and hop radius. This should enable the creation of a minimum viable product, combining Dijkstra's algorithm with existing imaging algorithms.

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Mechanical metastructures with integrated flexible electronics systems

Rujie SunStudent: Rujie Sun
Supervisors: Fabrizio Scarpa, Jonathan Rossiter, Ian Farrow and John Rogers (University of Illinois at Urbana-Champaign, USA)

With an increasing aging population and emphasis on maintaining acceptable health and living conditions for the elderly population, it is important to identify technologies that support the sustainability of existing medical systems. In the UK today, there are 10.8 million disabled people, and nearly 6.5 million have mobility impairments; 6 million have an impairment for lifting and carrying weights and 2.4 million have impaired co-ordination. Growing healthcare is in need of more advanced therapies.

Biointegrated devices with health monitoring systems are regarded as one potential solution to mitigate the above concern. Recorded data from these devices is important to predict potential health issues. Recent advancements in material science, manufacturing techniques and miniaturization of electronics have facilitated the development of multifunctional materials that have the potential to couple sensing, actuation, computation, and communication. Next-generation wearable devices will integrate compliant sensor systems to monitor body health (e.g., temperature, strain, blood-pressure, and heartbeat), memory modules to store recorded data during continuous, long-term monitoring, battery modules, wireless power/data transmission, actuation parts to deliver feedback and physical therapy. Current emerging electronics technologies for the human body are classified in three categories: soft, injectable, and bioresorbable electronics. The surfaces of most organs of the human body are soft and curvilinear and require low-stiffness epidermal electronics to enable intimate yet imperceptible deformation conformal to their surfaces. Injectable electronics are extensions of current surface-mounted devices. Such techniques make it possible that soft bioelectronics could be noninvasively delivered into deep regions of biological systems. Biocompatibility is an essential element for human body-based device, especially implantable ones. Desirable biointegrated electronics should be dissolved readily in a controlled way to reduce side effects to minimal level.

The aim of this project is to develop a soft, stretchable, and conformable biointegrated device for body pressure measurement. The following specific objectives will be investigated:

  • Stretchable interconnect designs to at least meet the requirement of 30% stretchability as human skin;
  • Human-machine interface designs to mitigate the mismatch between soft skin and hard electronics;
  • Pressure sensor design in terms of the sensitivity improvement;
  • System integration design and test, and combination with wireless communication components to evaluate the performance of the whole system.

This research shows potentials in the area of individual healthcare and is well suited to the change of future lifestyles:

  • Easy access to real-time health status under various conditions, such as sleeping, taking exercise;
  • Providing doctors with continuous, and long-term body data, reducing the possibility of misdiagnosis;
  • Contribution to the achievement of personalised treatment and remote medical technologies.

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A closed-loop recycling process for discontinous carbon fibre composites

Rhys TapperStudent: Rhys Tapper
Supervisors: Kevin Potter, Ian Hamerton, Marco Longana and HaNa Yu

It is becoming increasingly apparent in the composites industry that carbon fibre reinforced polymer (CFRP) usage growth rate is rising each year. From an original aerospace light-weighting commodity, CFRPs have expanded into multiple markets including high-performance vehicles, oil and gas, renewable energy and the mass-manufactured automotive sector. The prevalence of this weight saving agenda is embodied by the current material paradigm shift occurring in the automotive market. Adoption of CFRP material has not been a seamless transition into the automotive market due to substantial barriers. The most influential of these being high carbon fibre (CF) costs and long manufacturing cycles for mass-manufacturing applications. A vehicle's environmental impact is not solely governed by its emissions characteristics but, as CFRPs are adopted, the energy associated with material production carries almost as much of the burden. Materials production impact i.e. material manufacturing and component production waste, is mainly influenced by raw material manufacturing costs which is dominated by the production of CFs. This is not easily optimized as it depends heavily on the price of oil and the CF manufacturing process. Component production waste is an inevitable consequence and can never be fully eradicated therefore a reduction in the environmental penalty of CFs must be provided elsewhere. One route to increasing the desirability of CFRPs is in the recycling of used components and production wastes. As seen previously, it is difficult to reduce the economic and energy costs associated with virgin carbon fibres (vCFs) therefore the development of processes which can utilize these fibres, thus giving them a second lifetime, will serve to decrease their overall environmental impact.

The aim of this project is to develop a fully closed-loop recycling process for industry quality composite materials. The core element of the process includes the use of an automated alignment machine which takes dispersions of short carbon fibres and lays them into preforms using convergent flow and momentum change to confer a high degree of alignment. Short carbon fibres are used because their geometries make them intrinsically recyclable. Thermoplastics resins will be used for the matrix material as the lack of cross-links in the polymer structure make them ideal candidates for recycling also. As the alignment process has already been validated and proven to provide high-quality specimens using thermosets, it is the objective of this project to take the aligned prepregs, separate the constituents, and reform the prepreg with limited or no mechanical property depletion. The collection, separation and reformation methods must fit within a system that can be defined as 'closed-loop'. A closed-loop process is one that, after the incorporation of some initiator materials i.e. matrix and reinforcement at the beginning of the cycle, requires no additional material input for repeated cycles.

If the process is successful in producing highly aligned, high quality prepregs for use in industry, such as the automotive industry, it will be providing them with a lightweighting material of desirable properties without the significant drawbacks of undesirable economic and environmental costs. A comprehensive Life Cycle Assessment will be carried out in order to fully understand the energy costs associated with the proposed process. This will help elucidate whether the system is in fact closed-loop and will aid in defining the true benefit of the process in its chosen industrial context.

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Nanostructure modelling for nanocomposite materials

Mat TolladayStudent: Mat Tolladay
Supervisors: Fabrizio Scarpa, Dmitry Ivanov and Neil Allan

Interest in nanotubes and their potential applications has been growing ever since their initial prediction over two decades ago. Concurrently, there has been development of micro, meso and nano scale machines looking to provide new technological capabilities in many different research fields. Components of these miniature machines are an obvious application for nanotubes, however new methods for accurately modelling such machines are required. The standard methods for macro scale modelling rely on the assumptions of continuum mechanics whereas at smaller scales the molecular structure of the materials becomes apparent leading to unacceptable inaccuracies. Although computational chemistry has produced tools for modelling molecular structures they normally require significant computational resources when attempting to model large systems of atoms. In an attempt to overcome this issue a method has been developed that combines aspects from the techniques developed for use in computational chemistry and the methods used for structural analysis that are commonplace in engineering.

This new method has the potential to increase the speed of simulations of molecules and their interactions. Such high speed simulations would make it easier for simulating long chain polymers, nanotubes, fibres and interactions between these components in a composite material.

The purpose of this project is to examine the efficacy and accuracy of this method and to improve and where possible extend it to incorporate more complex behaviours such as thermal and piezoelectric responses of the materials.

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Effects of voids on the interlaminar failure of carbon/epoxy composites

Iryna Tretiak Student: Iryna Tretiak
Supervisors: Luiz Kawashita, Stephen Hallett, Robert Smith and Jonathan Taylor (Rolls-Royce)

The processes used in the manufacture of composite structures have the potential to introduce different defects, such as porosity, fibre and ply misalignment, fibre waviness, foreign object inclusions, partially-cured matrix material, resin rich areas and delamination. Arguably the most crucial defects are voids, as they are difficult to eliminate during manufacturing, can induce other defects such as delamination and have a detrimental effect on the mechanical properties. The elimination of voids during high-volume production is almost impossible and so it is essential to understand and consider its effect on the properties, so that the structure can be designed with this in mind (e.g. by means of a knock-down factor). There are a number of theories that describe the relationship between mechanical properties and void content under various loading conditions. However these models are not definitive as they need to consider, for example, different types of material, different stacking sequences and processing parameters that could affect the distribution, location, shape and size of voids. Currently, no model is able to satisfactorily consider all these variables and produce an accurate assessment of the effects upon the properties of the material.

The aim of this research is to conduct the experimental and modelling investigation into methods to predict the failure from voids using finite element analysis techniques, advanced testing and characterisation methods.

Most of the information in the literature on how defects influence the performance of components is based on empirical testing. This relies on being able to reproduce defects observed in production in a controlled laboratory environment or expensive testing of full scale components. Work on numerical prediction of voids on performance is more limited, using techniques such as cohesive elements to model voids as micro-delaminations or modelling the voids explicitly at the microscale. Industrial approaches are much more simplistic, using a knockdown on design stresses based on void percentage. This does not take into account the location and size of individual voids, thus small distributed defects can be considered the same as one large void.

The work in this project aims to review the available methods for modelling of porosity-type defects, and develop new approaches for prediction of their influence on failure, e.g. via advanced finite element modelling, and validate these methods through experimental characterisation and testing.

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Effect of direct deposition of dry particle tougheners on the processability and forming quality of carbon fibre/epoxy prepregs

Logan WangStudent: Logan Wang
Supervisors: Byung Chul (Eric) Kim, Kevin Potter and HaNa Yu

Drape forming is a cost-effective composite thermoforming method that was originally developed for thermoplastic sheet forming. Now it has been used to manufacture large thermosetting composite structures with relatively less complex geometries. This technique improves production rate by laying multiple prepregs to create a flat uncured laminate first and then forming it into a desired shape later instead of the time consuming direct lay-up process. However, the disadvantage of this method is that out-of-plane wrinkling or fibre buckling occurs during forming, which makes this method challenging.

This undesirable defect is mainly generated by high interfacial friction between the plies due to the tacky surface resin preventing ply slippage. Therefore, the forming process usually needs to be carried out at an elevated temperature to enhance the ply slippage by reducing the resin viscosity, but the cost of heating energy and processing time can be significant when manufacturing large aircraft components, such as wing spars. Additionally, the reduced resin viscosity could result in lower fibre bonding strength, which could cause fibre splitting and distortion during forming.

The aim of this project focuses on improving drape forming quality of CFRP laminate by interleaving lubrication materials at laminate interfaces to reduce the interply friction, and the lubrication materials would expect to improve the interlaminar strength of composites after consolidation. The preliminary study has proven this new forming method effectively decreased prepreg interply frictional resistance at temperature lower than the commercial hot drape forming temperature, and the quality improvement was demonstrated by forming composite C-sections. The effect of interleaving lubrication materials on the interlaminar shear strength of the cured laminate was also studied.

Further study is focused on the lubrication effect and mechanical properties of different interleaving materials, and the influence of process conditions on laminate performance. The long-term aim of this research is using the above concept to manufacture more complex and high-quality CFRP parts cost-efficiently, and make it ready for industry applications.

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